The invention generally relates to microwave devices, and, more particularly, to high-Q microwave resonant cavities.
Devices that manipulate microwave radiation often include metallic components having surfaces that reflect the radiation. For example, microwave resonant cavities confine a microwave electromagnetic field by reflecting the field from the conductive walls of the cavity. Such cavities have a variety of applications, for example, filters, oscillators, frequency meters, tuned amplifiers and accelerometers.
The shape, dimensions and chemical composition of the metallic components of a device can have a substantial effect on the behavior the microwave radiation. For example, deformation of a resonant cavity, or perturbation of an object in the cavity, will perturb the electromagnetic waves in the cavity, and thus cause a change in the resonant frequency of the electromagnetic normal modes. Such effects can be beneficially utilized, for example, in accelerometers that are based on resonant cavities. Reflective losses, however, can limit the sensitivity of accelerometers.
Devices fabricated from highly pure metal can have surfaces that efficiently reflect microwave radiation, though pure metals will generally have poor thermomechanical stability. A stable metal alloy or ceramic can be used in conjunction with a metal coating; however, many prior art coating methods are limited in their ability to produce coatings of a desired purity, thickness or structural uniformity.
For example, electrochemical deposition (e.g., plating) can provide a metal coating on a conductive substrate. This deposition method can produce relatively thick layers, but the layers are generally impure and porous. Other deposition methods can provide a highly pure metal layer on conducting or non-conducting substrates. Such methods, however, are generally limited to the formation of very thin films, and are limited in their ability to provide uniform coatings, particularly when line-of-sight is unavailable for all surfaces of interest.
Resonant cavities have been manufactured from superconducting materials to obtain high-Q cavities for extremely sensitive accelerometers. Unfortunately, superconducting materials present manufacturing and operational difficulties, can be expensive, and are impractical for general applications.
The invention involves microwave devices that include highly efficient reflecting surfaces provided by conductive fittings bonded to substrates. The invention can provide, for example, high-Q microwave cavities. High-Q cavities in turn enable, for example, highly sensitive accelerometers.
More specifically, the invention involves devices, and methods for manufacturing devices, that have a preformed metal fitting bonded to a substrate. Forming a fitting prior to bonding the fitting to a substrate facilitates use of high-purity, low-resistivity metals. The substrate can thus be formed from any material that is structurally convenient for microwave device use, though it may have a poorly reflecting surface. For example, ceramic substrates can provide excellent rigidity and thermal stability, but are electrically insulating and thus do not reflect microwave radiation. Further, the invention enables the use of substrates having shapes that would make coating with high purity metals difficult with many prior art methods.
By bonding a sufficiently thin metal fitting to the substrate, the thermomechanical benefits of the substrate are obtained in conjunction with the efficient reflectivity of a low resistivity metal fitting. Reducing the resistivity of a fitting, for example, by increasing the metal purity, enhances the benefits of the invention by increasing the efficiency of reflection.
The invention thus solves problems found in prior art microwave devices. The invention provides fittings that can have a highly pure and highly uniform composition throughout their thickness. The fittings can be attached to a variety of substrate surfaces. An initial fitting thickness can be selected to accommodate manufacturing steps that occur prior to bonding, and the fitting can be thinned after bonding to a desired final thickness.
Accordingly, in a first aspect, the invention features a device for manipulating microwave radiation. The device includes a substrate that defines the shape of a surface for reflecting microwave radiation. The substrate can define the shape, for example, of a microwave resonant cavity or a component that, more generally, reflects microwave energy. The device also includes a metal fitting conforming to the defined shape. The metal fitting provides the surface that reflects microwave radiation.
The metal fitting is preferably formed of a high purity metal, such as high purity copper, silver or aluminum. Bulk samples of metal, from which fittings can be fashioned, may be fabricated, for example, from a wrought metal sample. The metal sample can be prepared by casting, and by cold or hot working the metal. The fitting may consist of a metal that is at least 99% pure.
The device can be any of a variety of devices that manipulate microwave energy. Such devices include, for example, a microwave resonant cavity, microwave waveguide or a microwave reflector.
The metal fitting preferably has a thickness of greater than 10 xcexcm after completion of fabrication of the device. The thickness of the metal fitting is generally less than 500 xcexcm, and preferably less than 100 xcexcm. These thicknesses can limit the effect of the fitting on the size and shape of the device during thermal cycling.
In preferred embodiments, the substrate includes an insulator, such as a ceramic. A ceramic can provide a low coefficient of thermal expansion, and thus provide stable device dimensions during thermal cycling. The substrate can control the thermal behavior of the device dimensions when a relatively thin metal fitting is used.
The fitting can be bonded to the substrate via a variety of means. For example, a braze joint or an adhesive, for example, an epoxy, can be utilized. Alternatively, an interference fit, or compression fit, may be used to provide a bond via friction. Further, a combination of bonding means may be used.
The metal fitting can have a machined surface. The fitting may cover all or part of surfaces that are exposed to microwave energy.
In a second aspect, the invention features a method for making a device for manipulating microwave radiation. The method includes providing a substrate that defines a shape of a surface for reflecting microwave radiation. A metal fitting, which has a sufficient thickness to provide mechanical stability, is provided. The metal fitting is bonded to the substrate, and provides the surface that reflects microwave radiation.
The metal fitting can be thinned after bonding it to the surface, for example, via machining. Milling can also be used to shape the metal fitting prior to bonding it to the substrate. An interference bond can be obtained by cooling the metal fitting, placing the metal fitting adjacent to the substrate and causing the metal fitting to warm to an original temperature.
Similarly, a bond can be obtained by heating the substrate, placing the metal fitting adjacent to the substrate and causing the metal fitting to cool to an original (e.g., room) temperature.
Adhesives can be used to assist or provide bonding. Pressure may be applied to the metal fitting to obtain a thinner adhesive layer and/or to deform the metal fitting to conform to a surface of the substrate.